PROJECT SUMMARY Breast cancer is the most prevalent cancer among women and encompasses a group of different diseases as a result of the many different subtypes of breast cancer cells. The ER/PR/Her2 triple negative breast cancer (TNBC) subtype is the most aggressive tumor type and has a poor prognosis. Thus it is imperative to identify and characterize novel therapeutic targets for the treatment of TNBC. While hundreds of genetic mutations are known in promoting cancer, long non-coding RNAs (lncRNAs) are emerging as critical components of the epigenetic regulation of normal cellular functions and have been associated with several cancers. LncRNAs function by forming interactomes with chromatin, many classes of proteins (including transcriptional regulators), and other RNA molecules, to regulate both genome structure and gene expression. They have been identified as diagnostic and/or prognostic factors associated with different breast cancer subtypes and grades. However relatively few have been functionally and mechanistically characterized, which is a requirement for developing future intervention strategies to prevent TNBC progression. Preliminary data has identified specific lncRNAs associated with distinct cancer cell subtypes. This proposal focuses on understanding the mechanism of a novel lncRNA, MANCR that is selectively expressed in TNBC cells. Our published findings have shown that MANCR inhibition results in increased genomic instability and tumor cell death in TNBC. This proposal addresses the hypothesis that MANCR expression supports genomic stability of TNBC cells and that the mechanisms contributing to this effect can be identified through direct characterization of the lncRNA interactome. To test this hypothesis, three specific aims will address both mechanistic and preclinical investigations into this lncRNA. Specific Aim 1: Determine the regulation of MANCR expression in TNBC by RUNX2 and AP-1 regulatory elements. Specific Aim 2: Establish mechanisms for the role of MANCR in TNBC cells that induce regulatory factors involved in maintaining genomic stability of TNBC. Specific Aim 3: Demonstrate that this lncRNA affects tumor growth in vivo. Impact: These studies will, for the first time, identify the regulatory mechanisms of a novel lncRNA associated with TNBC. Our findings will greatly impact on its potential as a therapeutic target because MANCR is absent in nearly every normal tissue, thus limiting off-target effects as a therapeutic agent. We anticipate that knowledge of the molecular and pre-clinical findings of MANCR will be the basis for future studies in the design of therapies to treat patients with TNBC.